ABSTRACT. Cytochrome P450 (CYP) 3A4 is the most abundant CYP enzyme in the human liver, and it metabolizes ~60% of known drugs. CYP3A4-mediated drug metabolism is impaired in patients with infections, diabetes, cancer, cardiovascular diseases, liver disorders and many other diseases. Disruptions in drug metabolism in diseases are associated with induction of inflammatory markers and reductions in expression of CYP enzymes. Thus, in most patients, medications are exposed to a micro-environment where inflammatory mediators are activated. This increases the risks of drug-drug interactions and adverse drug reactions in these patients. The objective of this exploratory proposal is to perform genome-wide mapping and bioinformatics analysis to identify key regulators involved in down-regulation of human CYP3A4 enzymes in vivo. CYP3A4 gene expression is regulated by basal transcription factors as well as nuclear receptors (NRs). In vitro studies have shown that CYP3A4 expression is also regulated by microRNAs (miRNAs). Thus, down-regulation of CYP3A4 enzymes may be a cumulative effect of transcriptional and post-transcriptional modifications by transcription factors, NRs and/or miRNAs. Our central hypothesis is that down-regulation of CYP3A4 expression is controlled by transcription factor/NR-associated epigenetic modifications at the target chromatin as well as by changes in miRNA binding to CYP3A4 mRNA. The first aim of this proposal will examine the hypothesis that down- regulation of CYP3A4 gene is controlled by transcription factor/NR-associated epigenetic modifications at the target chromatin. The second aim will determine the role of miRNAs in down-regulation of CYP3A4 enzymes in vivo. CYP3A4 expression can be activated by diverse chemicals which induce NRs, including pregnane X receptor (PXR). On the other hand, CYP3A4 can be down-regulated by inflammatory mediators including cytokines and the bacterial endotoxin, lipopolysaccharide (LPS) which are associated with diseases. To identify the key regulators involved in alteration of CYP3A4 enzymes, our strategy is to utilize a combined approach of CYP3A4 induction by PXR ligand (e.g. PCN) and CYP3A4 down-regulation by LPS. The proposed studies using genome-based approaches can unravel novel regulatory elements which contribute to down-regulation of human CYP3A4 enzymes in vivo. These regulators can then be targeted to prevent undesirable effects of drugs due to changes in CYP3A4-mediated drug metabolism. Ultimately, this can lead to the development of new strategies to improve the safety of medications in individual patients.